I am having trouble with tunnels in white strong & flexible. If I print tunnels with .45mm diameter (which should give a hole with respect to the 0.2mm precision), they resolve (second try) to a depth of maximal 3-4mm. Deeper in the material they do not exsist any longer (we cut it open). Has anybody experience which minimal tunnel diameter to use for long tunnels ~ 2cm lenth? I think these parameter would also belong into the materials sheet...

SLS shrinks. There's no avoiding that. Obviously there's a scale factor involved (2-4% depending on axis, material, alignment of jupiter with my little toe, etc) which the SLS operator has programmed into the build, but there's also an offset factor, to combat shrinking wall sections and powder granule size and other stuff I can't remember. This offset will be around 0.2mm on both the X and Y. Its not as simple as this, one has to also take beam diameter into account, but the gist here is your hole is actually being built smaller than it should be, and its entirely possible the unsintered material inbetween has heated to the point where it has solidified.

Your best bet in trying to achieve holes this small is drilling them - it will be of a far finer tolerance that way.